Inorganic Chemistry, Vol. 17, No. 10, 1978 2809
Five-Coordinate Platinum(I1) Carbonyl Complexes W. H. Atwell and D. R. Weyenberg, Angew. Chem., Int. Ed. Engl., 8, 469 (1969).
P. P. Gaspar and B. J. Herold, in “Carbene Chemistry”, W. Kirmse, Ed., 2nd ed, Academic Press, New York, N.Y., 1971. J. C. Thompson and J. L. Margrave, Science, 155, 669 (1967). J. L. Margrave and P. W. Wilson, Acc. Chem. Res., 4, 145 (1971). P. S.Skell and E. J. Goldstein, J . Am. Chem. Soc., 86, 1442 (1964). M. A. Nay, G. N. C. Woodall, 0. P. Strausz, and H. E. Gunning, J . Am. Chem. Soc., 87, 179 (1965). J. H. Purnell and R. Walsh, Proc. R. Soc. London,Ser. A, 293,543 (1966). I. DuBois, G. Herzberg, and R. D. Verma, J . Chem. Phys., 47,4262 (1967). P. Estacio, M. D. Sefcik, E. K. Chan, and M. A. Ring. Inorg. Chem., 9, 1068 (1970). M. Bowrey and J. H. Purnell, J . Am. Chem. Soc., 92, 2594 (1970). P. S. Skell and P. W. Owen, J . Am. Chem. Soc., 94, 5434 (1972). V. M. Rao, R. F. Curl, P. L. Timms, and J. L. Margrave, J . Chem. Phys., 43, 2557 (1965). P. L. Timms, R. A. Kent, T. C. Ehlert, and J. L. Margrave,J . Am. Chem. SOC., 87,2824 (1965). H. P. Houkins, J. C. Thompson, and J. L. Margrave, J . Am. Chem. Soc., 90,901 (1968). J. C. Thompson and J. L. Margrave, Inorg. Chem., 11, 913 (1972). P. P. Gasoar. B. D. Pate, and W. C. Eckelman, J . Am. Chem. Soc., 88, 3878 (1966). P. P. Gaspar, S. A. Bock, and W. C. Eckelman, J. Am. Chem. Soc., 90, 6914 (1968). P. P. Gaspar, S. A. Bock, and C. A. Levy, Chem. Commun., 1317 (1968). P. P. Gaspar and P. Markusch, Chem. Commun., 1331 (1970). P. P. Gaspar, P. Markusch, J. D. Holten 111, and J. J. Frost, J . Phys. Chem., 76, 1352 (1972). Y.-N. Tang, G. P. Gennaro, and Y. Y. Su,J . Am. Chem. Soc., 94,4355 (1972). G. P. Gennaro, Y.-Y. Su,0. F. Zeck, S.H. Daniel, and Y.-N. Tang,
J . Chem. Soc., Chem. Commun., 637 (1973). (24) P. P. Gaspar, R.-J. Hwang, and W. C. Eckelman, J . Chem. Soc., Chem. Commun., 242 (1974). (25) 0. F. Zeck, Y. Y. Su,G. P. Gennaro, and Y.-N. Tang, J . Am. Chem. Soc., 96, 5967 (1974). (26) 0.F. Zeck,Y. Y. Su,and Y.-N. Tang, J. Chem. Soc., Chem. Commun., 156 (1975). (27) 0. F. Zeck, Y. Y. Su,G.P. Gennaro, and Y.-N Tang, J . Am. Chem. Soc., 98, 3474 (1976). (28) Preliminary information of this work has been reported in, R. A. Ferrieri, E. E. Siefert, M. J. Griffin, 0. F. Zeck, and Y.-N. Tang, J . Chem. SOC., Chem. Commun., 6 (1977). (29) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S.Rowland, Anal. Chem., 34, 741 (1962). (30) T. H. Chao, S. L. Moore, and J. Laane, J. Organomet. Chem., 33, 157 (1971). (31) For information concerning the copyrolysis of disilanes see, (a) E. M. Tebben and M. A. Ring, Inorg. Chem., 8, 1787 (1969); (b) P. Estacio, M. D. Sefcik, E. K. Chan, and M. A Ring, ibid., 9, 1068 (1970); (c) E. A. Chernyshev, N. G. Kamalenkov, and S.A. Bashkerova, Zh. Obshch. Khim., 41, 1175 (1971). (32) J. M. Bassler, P. L. Timms, and J. L. Margrave, Inorg. Chem., 5, 729 (1966). (33) E. P. Blanchard and H. E. Simmons, J. Am Chem. Soc., 86,1337 (1964); M. Jones, Jr., W. J. Baron, and Y. H. Shen, ibid., 92,4745 (1970); U. Burger and R. Huisgen, Tetrahedron Lett., 34, 3057 (1970). (34) P. S. Skell and E. J. Goldstein, J. Am. Chem. Soc., 86, 1442 (1964). (35) For a review of methylene reactions and references see, “Carbene Chemistry”, W. Kirmse, Ed., 2nd ed, Academic Press, New York, N.Y., 1971, Chapters 8 and 9. (36) J. Graefe, M. Miihlstadt, and P. Kuhl, 2. Chem., 10, 192 (1970); 9, 23 (1969); Tetrahedron Left. 3431 (1969); H. Nozaki, M. Kawanisi, and R. Noyori, J. Org. Chem., 30, 2216 (1965).
Contribution from the Department of Chemistry, University of Delaware, Newark, Delaware 1971 1
Substituent Effects upon Dissociation and Migration Reactions of Five-Coordinate Platinum(11) Carbonyl Complexes NOBUYUKI SUGITA,? JOSEPH V. MINKIEWICZ, and RICHARD F. HECK*
Received March 9, 1978 A series of five iodobis(triphenylphosphine)arylplatinum(II) complexes was prepared and rates of carbonylation in the presence and absence of excess triphenylphosphine were measured. Four related palladium(I1) complexes were compared with the platinum complexes. The results are interpreted in terms of a five-coordinated carbonyl intermediate which forms the acylmetal product by two reaction paths-a migratory route and a dissociative route. The migratory reaction is much more sensitive to changes in the electronic character of the migrating group than the dissociative reaction is. The results show that carbon monoxide insertion is not rate limiting in the catalytic carbonylation of aryl halides.
The carbon monoxide insertion reaction is one of the basic reactions of organo-transition-metal compounds. Knowledge of the mechanism of this reaction would be useful in understanding how different ligands affect the reaction course. This information ultimately could lead to new catalysts with optimum activity for various carbonylation reactions. We undertook a study of the reaction of halobis(triorganophosphine)organoplatinum, -palladium, and -nickel complexes with carbon monoxide with the above goal in mind.’ With the platinum complexes the reaction was shown to involve a preequilibrium step forming a fluxional five-coordinate carbonyl complex, 11, which then reacted by two paths to form the acylplatinum product V. One of the two paths involved a direct migration of the organic group from platinum to the carbon monoxide ligand and the other a dissociation of one of the triorganophosphine groups. In the second process fluxional intermediate I11 was believed to be formed which then underwent a migration to three-coordinate species IV. A final reassociation of a phosphine ligand gives the observed product. Numerous metal complexes were studied and ‘On leave from the Institute for Chemical Rasearch, Kyoto University, Kyoto, Japan.
0020-1669/78/1317-2809$01.00/0
Scheme I PPh3
I
t CO
R-Pt-I
I
K1
e I(PPh3)2(CO)(R)Pt I1
PPh3
fl .PPhS
h2
1k4
I V
PPh-
V
considerable knowledge of ligand effects in this reaction was acquired. We now report additional data bearing o n how various aryl substituents influence the relative preference of the five-coordinated intermediate for the two reaction paths 0 1978 American Chemical Society
Sugita, Minkiewicz, and Heck
2820 Inorganic Chemistry, Vol. 17, No. 10, 1978
z
10
9
9
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1 i d
I
I
9
.o
*
1
7
d
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2
4
3
r.
i 4
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7
3
P
P
N
2
2
-Y
9
9
x
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2
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I
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I
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Io
W I-
9 i
N
1 ii
N
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i
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m 7
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0
Y
b, I
z 3
1
* 0
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6
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Inorganic Chemistry, Vol. 17, No. 10, I978 2811
Five-Coordinate Platinum(I1) Carbonyl Complexes '?
N
4
a
Table IV. Relative Rates of Reaction of Platinum Complexes with CO at 30.0 "C in Tetrachloroethane Solution at Atmospheric Pressure
;
R in RPt(PPh,),I
I
a
t
[':
2
v)
c? 3
b
P
Q
27Q
Q
-c1 -b3b 3 b x 3o 4b 3b
R b
e
i
s
i *
2 2
!j
4CH,OCOC6H,WIC,H,I
_
C6H5-
4CH,0C.HAWH;C,H,-2-CH30COC6H,-
K,k, ,C atm-I s-'
K, k, $ atm-I s-,
1.OQ
1.0a
1.9" 5.3 5.4 5.6
1.9" 15 35
O.lb
40 4.4b
dissociation/ migration 32 32 12 5 5 0.7b
a Data calculated from extrapolated rate constants. CalculaRelated at 40.0 "C. Relative rate of dissociation of PPh,. tive rate of migration of R to CO.
e! a
8
y! d
1
I
and some additional information on the reactions of the related palladium(I1) complexes.
I
Results
9)
d
BE -2 *
1
4
E w P
E' E
0
a
. 8 3 w v)
5 c, e,
a
5i U
Platinum Complexes. Five iodobis(tripheny1phosphine)arylglatinum(I1) complexes were reacted with carbon monoxide, and the reaction rates were measured by gas volume changes in tetrachloroethane solution in the presence of and in the absence of (excess) triphenylphosphine. The data obtained are given in Table I. Good first-order kinetics were observed in all cases between about 5 and 85% reaction. Attempts to use a 4-nitrophenyl derivative were abandoned since the reaction was too slow to observe. In all instances the reaction products were shown to be the corresponding acylplatinum complexes both by the vco at 1600-1630 cm-' and by the fact that very close to 1 equiv of CO was absorbed/mol of platinum complex reacted. In these examples with triphenylphosphine ligands there was no clearly detectable accumulation of the five-coordinate intermediate. All reactions were carried out with a series of concentrations of triphenylphosphine to determine the maximum depression of the reaction rate achievable. Palladium Complexes. The much higher rates of carbonylation of arylpalladium complexes prevented us from obtaining as much data as we did with the platinum compounds. However, substituent effects appeared to be similar. The results are summarized in Table I1 with some pertinent results obtained previously. Only two values of the ratio of rates of the dissociation to migration paths are known. They are the value of 32 obtained at 2.3 "C for bromobis(tripheny1phosphine)phenylpalladium(II)' and a value of 4.6 at 43.2 OC obtained for the bromobis(triphenylphosphine)(4-(trifluoromethyl)phenyl)palladium(II) complex.' The corresponding iodophenylplatinum complex had a value of 12 at 30 OC.' It has been suggested that the carbon monoxide insertion reaction of arylpalladium complexes is involved in the palladium-catalyzed alkoxycarbonylationZand amidation react i o n ~ .Therefore ~ it was of interest to see if substituent effects in the catalytic reaction paralleled those of the stoichiometric carbonylation. Catalytic reactions of 4-iodoanisole, iodobenzene, and methyl 4-iodobenzoate were carried out with carbon monoxide, tri-n-butylamine, and benzyl alcohol in dimethylacetamide as solvent. Amidations with benzylamine were also carried out with the first two halides. The results are summarized in Table 111. The first-order rate constants showed rather large average errors; however, the trends are clear. The substituent effects in the catalytic reaction are remarkably small; all reactions occurred at rates within a factor of 5 of each other and no correlation with the stoichiometric reaction was apparent. The carbonylation of one platinum complex, iodobis(tri-
2812 Inorganic Chemistry, Vol. 17, No. 10, 1978
Sugita, Minkiewicz, and Heck
Table V. Properties of Platinum Complexes Prepared anal. mp,"C
%yield
calcd
found
calcd
found
calcd
found
v n n nf u u -carbonylated product. cm- *
>250 >250 >250 >250 >230
93 41
5 3.8 3 52.65 54.61 54.15 55.08
5 3.84 52.89 54.64 54.29 55.16
3.79 3.58 3.82 3.91 3.98
3.90 3.68 3.83 4.06 4.06
12.93 13.24 13.74 13.31 13.53
12.90 12.73 13.90 13.12 13.12
1600 1610 1620 1625 1630
%H
%C
R in R(I)PW'h,), 4 C H ,OCOC,H,CClC,H,CA4-CH,0C6H,CCH,C,H,-
ao
68 60
phenylphosphine)-4-anisylplatinum(II), was shown to be first order in carbon monoxide as expected but not established previously. Discussion of Results The results of the stoichiometric platinum carbonylations are best discussed in terms of the mechanism proposed previously' (Scheme I). The observed reaction rates can be expressed in terms of the individual steps K1' = K1[CO] = [II]/[I]
(1)
d[III]/dt = k,[II] - k-3[III][PPh3] - k4[III]
(2)
d[IV] /dt = k,[III] - ks[IV] [PPh3]
(3)
d[COI d[II -dt dt [I], = [I] [11] [111] [IV]
(4)
+
+ + + [VI [PPh3] = [111] + [IV] + [PPh3]o
(5)
(6)
where suffix 0 denotes the initial concentration. If it is assumed that ks >> k4 and the steady-state approximation is made for [11], [111], and [IV], then [IV] must be negligible compared with [111] [PPh310and eq 6 reduces to [PPh3] x [111] [PPh310. If the steady-state approximation holds, then [11] [111] [IV] must be small compared with [I] + [VI and [II0 = [I] [VI. It follows that
+ +
+
+
+
d[COI d[II =-dt dt
X-
d[VI = k4[III] dt
+ k2[II]
Substituting [11] = K,'[I] and letting d[III]/dt = 0 [111] =
4Kl'k3k-3 [ I]) '/2) From the fact that 4Kl'k3k-3[I]/(k-3[PPh3]o + k4) = it follows that (k-3[PPh3]o+ k4)2>> 4K4'k3k-3[I] and [111] = K1'k3[I] /(k-3[PPh3]0 k4); therefore
+
or
%I
kobsd
[
52:k3-[
k4 + K'k,]
The rate constants for the triphenylphosphine suppressed rates then are Klk2. The relative values are listed in Table IV. They represent rates of migration of the aryl group from platinum to carbon monoxide. The difference between these values and the unsuppressed rate constants is the rate constant for the triphenylphosphine dissociation path leading to product, essentially K,k3. The ratios of the dissociative rate constants to the migratory rates are k3/k2. These values are listed in Table IV along with relative rate constants for both reaction paths. It is clear that rates of dissociation are considerably less sensitive to changes in the electronic character of the cr-bonded aryl group than the migratory rates are. The selectivity of dissociation relative to migration decreases from 32 to 5 as the para substituent becomes more electron donating going from 4-carbomethoxy or 4-chloro to 4-methoxyl or 4-methyl. A plot of log (Klk2) against the u value gives a p for the aryl migration reaction of -3.6 a t 30 "C. The aryl group may be migrating as an anion as in the 1,2 shifts in carbon compounds but with much smaller rate effects from the substituent. This cannot yet be concluded, however, since we do not know how changing the electronic character of the aryl group influences the composition of the equilibrium mixture of five-coordinate intermediate complexes. Undoubtedly, different isomers will show different preferences for the two reaction paths. If the transition state most resembles a square ~ y r a m i d 11 , ~ isomeric forms of the intermediate are possible. Of these it would seem reasonable to assume that only complexes with apical triphenylphosphine groups will dissociate and only isomers with cis aryl and carbonyl groups can undergo the migration reaction. Activation energies and entropies for the carbonylation reactions are shown in Table I. The values vary widely from compound to compound as might be anticipated from the complex nature of the reactions. Stoichiometric carbonylation data for one palladium complex, bromobis(triphenylphosphine)(4-nitrophenyl)palladium(II), is also given in Table I. The k3:k2value is 6.3 at 30 OC,about a fifth of the value 32 found for the platinum complexes with electron-withdrawing para substituents in the aryl group a t the same temperature. Other palladium complexes with less electron-withdrawing substituents were too reactive to be measured in our apparatus at temperatures where they had sufficient solubility. The absence of a correlation for different substituents between the stoichiometric CO insertion and the catalytic carbonylation of aryl halides demonstrates that the insertion of CO is not rate determining in the catalytic reaction if it is involved.
Experimental Section where [I] = [Il0 Thus
[CO]absorbed
and [CO] remains constant.
Materials. Tetrachloroethane (Aldrich) was washed with aqueous potassium carbonate and with water until the washings were neutral
Inorganic Chemistry, Vol. 17, No. 10, 1978 2813
Electrophilic Cleavage of the Pt-C Bond and dried over molecular sieves. The preparation of iodobis(triphenylphosphine)(4-nitrophenyl)palladium was described previously.' Platinum complexes were prepared as described below. Triphenylphosphine was a product of the Ventron Corp. and it was used without further purification. Tetrakis(triphenylphosphine)platinum(O). To a solution of 6.55 g (0.025 mol) of triphenylphosphine in 50 mL of warm absolute ethanol was added under argon a solution of 2.59 g (0.005 mol) of chloroplatinic acid hexahydrate dissolved in 7 mL of absolute ethanol. After about 5 min of stirring, 15 mL of 85% hydrazine hydrate was added. The milky solution immediately turned yellow and the product crystallized. After the mixture was cooled to room temperature, the product was filtered and washed with warm ethanol, with cold water, and finally with cold ethanol. After drying was done under reduced pressure, 5.88 g (92%) of the product was obtained. General Procedure for Preparation of Platinum Complexes. A twoto threefold excess of aryl halide was stirred with a solution of the tetrakis(triphenylphosphine)platinum(O) complex in benzene (2.4 g in 35 mL was generally used). The clear solutions were stirred a t room temperature for about 3 h and heated at reflux temperature for about 2 h more. On cooling of the mixtures, the yellow-orange crystalline products were separated by filtration, washed with ether, and dried under reduced pressure. Yields were 40-90% of theory. Analyses and melting points are given in Table V. The vco observed in the carbonylation product is also listed (Table V).
Acknowledgment. This research was supported by a grant from the National Science Foundation. We thank Dr. N. Cortese for preparing most of the complexes and Matthey Bishop, Inc., for the loan of the chloroplatinic acid used in this study. Registry No. 4-CH,0COC6H4(I)Pt(PPh3)2, 67226-47-5; 4CIC6H4(I)Pt(PPh3)2, 67254-03-9; C6Hs(I)Pt(PPh3)2, 67254-04-0; 4-CH,OC6H,(I)Pt(PPh3)2, 67254-05-1; 4-CH,CbH4(I)Pt(PPh3)2, 67254-06-2; ~ - C H S O C O C ~ H ~ P ~ ( P 67226-48-6; P~~)~I, 402NC6H4Pd(PPh3)2Br, 67254-07-3; 4-O2NC6H4Pd(PPh3)2I, 67254-08-4; ~ - C H ~ O C O C ~ H ~ P ~ ( P67226-49-7; P ~ ~ ) Z I , 4-CH,OC6HJ, 696-62-8; C6H51, 591-50-4; 4-CH~OCOC6H4I, 619-44-3; 2CH,OCOC,H4I, 610-97-9; C ~ H S C H ~ O 100-51-6; H, C6HSCH2NH2, 100-46-9; PdC12(PPh3)2, 13965-03-2; CO, 630-08-0.
References and Notes (1) P. E. Garrou and R. F. Heck, J . Am. Chem. SOC.,98, 4115 (1976). (2) A. Schoenberg, I. Bartoletti, and R. F. Heck, J . Org. Chem., 39, 3318
(1974).
(3) A. Schoenberg and R. F. Heck, J . Org. Chem., 39, 3327 (1974). (4) A. D. English, P. Meakin, and J. P. Jesson, J . Am. Chem. SOC.,98,7590 ( 1976).
Contribution from the Istituto di Chimica Fisica, the University, Messina, Italy, and Centro Chimica Tecnologia Composti Metallorganici Elementi Transizione del C. N. R., Istituto di Chimica Industriale, the University, Padova, Italy
Mechanism of Electrophilic Cleavage of the Platinum-Carbon Bond in Platinum(11)-Diary1 Complexes R. ROMEO,* D. MINNITI, S. LANZA, P. UGUAGLIATI,* and U. BELLUCO Received October 16. 1977
A kinetic study is described of the electrophilic cleavage by the proton of one Pt-C u bond in complexes of type cis[Pt(PEt3)2(YC6H4)2](Y = p-NMe2, p-Me, p-OMe, H, m-OMe, p-F, p-C1, m-F, o-Me, o-Et, m-CF,) yielding cis-[Pt(PEt3)2(YC6H4)C1]and YC6H5in methanol and aqueous methanol. Electron-releasing substituents in the platinum-bonded aromatic rings increase the rates of electrophilic attack and a fairly good LFER is observed on plotting log k,, vs. the Hammett u parameter of the substituent Y in both solvents. Steric retardation occurs when the Pt-C bond is crowded by a neighboring ortho group. A large (ca. 6) kinetic isotope effect is observed on carrying out the PtC bond cleavage with DC1 in MeOD/D20 (90/10% v/v). The rates decrease with increasing water content of the solvent mixture. A mechanism is proposed which involves rate-determining direct attack of the proton on the Pt-C bond with release of YC6H5in a three-center transition by scavenging chloride state. The resulting transient intermediate may be either converted to ~is-[Pt(PEt~)~(Yc~H~)cl] in the absence of good nucleophiles (S = solvent). The mechanism ion (if present) or isomerized to tr~ns-[Pt(PEt~)~(Yc~H~)S]+ is discussed within the framework of general acidolysis of metal-carbon bonds in organometallic compounds.
Introduction The cleavage of non-transition-metal-carbon bonds (demetalation) has been extensively investigated and its mechanism elucidated in detail.' When alkyl groups are being cleaved in a bimolecular process, electrophilic substitution at the saturated carbon may take place via an open transition state (SE2). A cyclic mechanism (S,i) may also occur through X,-M-alkyl
+ E-N
-
-
6+ 6[X,-M...alkyl...E...N]' a1kYl-E -k N- + MX,+ ( S E ~ )
E = electrophilic end of the reagent N = nucleophilic end
}
*TOwhom correspondence should be addressed: R.R., Massina; P.U., Padova. 0020-1669/78/1317-2813$01.00/0
a cyclic transition state by concurrent attack of the two ends of the reagent on the polarized metal-carbon bond. This X,-M-alkyl
t E-N
-[
X,-M----alkyl
j E]* N- - --alkyl-€
t N-MX,
(S,I)
mechanistic view applies particularly to the cleavage of the metal-carbon bond in group 2B and 4B organometallics. In particular, for protonolysis reactions the driving force of the electrophilic attack is in any case the rate-determining proton transfer to the substrate. The extent of interaction of the proton with the alkyl group and of the nucleophile with the metal will depend on the charge separation being developed on both the cleaved group and the metal moiety, as well as 0 1978 American Chemical Society
